An NK1 receptor-dependent component of the slow excitation recorded intracellularly from rat motoneurons following dorsal root stimulation

Meta-analysis of biological variables' impact on spinal motoneuron electrophysiology data

Experimental, methodological, and biological variables must be accounted for statistically to maximize accuracy and comparability of published neuroscience data. However, accounting for all variables is nigh impossible. Thus we aimed to identify particularly influential variables within published neurological data, from cat, rat, and mouse studies, via a robust statistical process. Our goal was to develop tools to improve rigor in the collection and analysis of data. We strictly constrained experimental and methodological variables and then assessed four key biological variables within motoneuron research: species, age, sex, and cell type. We quantified intraexperimental and interexperimental variances in 11 commonly reported electrophysiological properties of spinal motoneurons. We first assessed variances without accounting for biological variables and then reassessed them while accounting for all four variables. We next assessed variances with all possible combinations of these four variables. We concluded that some motoneuron properties have low intraexperimental, but high interexperimental, variance; that individual motoneuron properties are impacted differently by biological variables; and that some unexplained variances still remain. We report here the optimal combinations of biological variables to reduce interexperimental variance for all 11 parameters. We also rank each parameter by intra- and interexperimental consistency. We expect these results to assist with design of experimental and analytical methods, and to support accuracy in simulations. Furthermore, although demonstrated on spinal motoneuron electrophysiology literature, our approach is applicable to biological data from all fields of neuroscience. This approach represents an important aid to experimental design, comparison of reported data, and reduction of unexplained variance in neuroscience data.NEW & NOTEWORTHY Our meta-analysis shows the impact of species, age, sex, and cell type on lumbosacral motoneuron electrophysiological properties by thoroughly quantifying variances across literature for the first time. We quantify the variances of 11 motoneuron properties with consideration of biological variables, thus providing specific insights for motoneuron modelers and experimenters, and providing a general methodological template for the quantification of variance in neurological data with the consideration of any experimental, methodological, or biological variables of interest.

Preoperative low-dose ketamine has no preemptive analgesic effect in opioid-naïve patients undergoing colon surgery when nitrous oxide is used - a randomized study

BACKGROUND

The analgesic properties of ketamine are associated with its non-competitive antagonism of the N-methyl-D-aspartate receptor; these receptors exhibit an excitatory function on pain transmission and this binding seems to inhibit or reverse the central sensitization of pain. In the literature, the value of this anesthetic for preemptive analgesia in the control of postoperative pain is uncertain. The objective of this study was to ascertain whether preoperative low-dose ketamine reduces postoperative pain and morphine consumption in adults undergoing colon surgery.

METHODS

In a double-blind, randomized trial, 48 patients were studied. Patients in the ketamine group received 0.5 mg/kg intravenous ketamine before surgical incision, while the control group received normal saline. The postoperative analgesia was achieved with a continuous infusion of morphine at 0.015 mg∙kg-¹∙h-¹ with the possibility of 0.02 mg/kg bolus every 10 min. Pain was assessed using the Visual Analog Scale (VAS), morphine consumption, and hemodynamic parameters at 0, 1, 2, 4, 8, 12, 16, and 24 hours postoperatively. We quantified times to rescue analgesic (Paracetamol), adverse effects and patient satisfaction.

RESULTS

No significant differences were observed in VAS scores between groups (P>0.05), except at 4 hours postoperatively (P=0.040). There were no differences in cumulative consumption of morphine at any time point (P>0.05). We found no significant differences in incremental postoperative doses of morphine consumption in bolus, except at 12 h (P =0.013) and 24 h (P =0.002). The time to first required rescue analgesia was 70 ± 15.491 min in the ketamine group and 44 ± 19.494 min in the control (P>0.05). There were no differences in hemodynamic parameters or patient satisfaction (P>0.05).

CONCLUSIONS

Preoperative low-dose-ketamine did not show a preemptive analgesic effect or efficacy as an adjuvant for decreasing opioid requirements for postoperative pain in patients receiving intravenous analgesia with morphine after colon surgery.

Substance P mediates excitatory interactions between striatal projection neurons

The striatum is the largest nucleus of the basal ganglia, and is crucially involved in motor control. Striatal projection cells are medium-size spiny neurons (MSNs) and form functional GABAergic synapses with other MSNs through their axon collaterals. A subpopulation of MSNs also release substance P (SP), but its role in MSN-MSN communication is unknown. We studied this issue in rat brain slices, in the presence of antagonists for GABA, acetylcholine, dopamine, and opioid receptors; under these conditions, whole-cell paired recordings from MSNs (located <100 microm apart) revealed that, in 31/137 (23%) pairs, a burst of five spikes in a MSN caused significant facilitation (14.2 +/- 8.9%) of evoked glutamatergic responses in the other MSN. Reciprocal facilitation of glutamatergic responses was present in 4 of these pairs. These facilitatory effects were maximal when spikes preceded glutamatergic responses by 100 ms, and were completely blocked by the NK1 receptor antagonist L-732,138. Furthermore, in 31/57 (54%) MSNs, a burst of 5 antidromic stimuli delivered to MSN axons in the globus pallidus significantly potentiated glutamatergic responses evoked 250 or 500 ms later by stimulation of the corpus callosum. These effects were larger at 250 than 500 ms intervals, were completely blocked by L-732,138, and facilitated spike generation. These data demonstrate that MSNs facilitate glutamatergic inputs to neighboring MSNs through spike-released SP acting on NK1 receptors. The current view that MSNs form inhibitory networks characterized by competitive dynamics will have to be updated to incorporate the fact that groups of MSNs interact in an excitatory manner.

Role of group II and III metabotropic glutamate receptors in rhythmic patterns of the neonatal rat spinal cord in vitro

Electrophysiological recordings were used to explore the role of group II and III metabotropic glutamate receptors (mGluRs) in oscillatory patterns generated by the neonatal rat spinal cord in vitro. Neither the group II agonist DCG-IV (and the selective antagonist EGLU), nor the group III agonist L-AP4 (and its selective antagonist CPPG) had any effect on lumbar motoneuron membrane potential or input resistance. This observation suggests that motoneurons expressed no functional group II and III mGluRs and received no network-based, tonic influence mediated by them. DCG-IV or L-AP4 strongly depressed synaptic responses evoked by single dorsal root (DR) stimuli, an effect counteracted by their respective antagonist. EGLU or CPPG per se had no effect on synaptic responses, indicating no mGluR autoreceptor-dependent control of transmitter release. L-AP4 largely depressed cumulative depolarization, windup and associated oscillations, whereas synaptic depression induced by DCG-IV waned with repeated stimuli. L-AP4 slowed down fictive locomotor patterns and arrested disinhibited bursting, which could, however, be promptly restored by DR electrical stimulation. DCG-IV had no significant effect on fictive locomotion, but it blocked disinhibited bursting. EGLU facilitated bursting, suggesting that burst termination was partly controlled by group II mGluRs. All these effects were reversible on washout. It is concluded that activation of group II and III mGluRs differentially modulated rhythmic patterns recorded from motoneurons via network-dependent actions, which probably included decrease in the release of neurotransmitters at key circuit points.

Windup of flexion reflexes in chronic human spinal cord injury: a marker for neuronal plateau potentials?

The physiological basis of flexion spasms in individuals after spinal cord injury (SCI) may involve alterations in the properties of spinal neurons in the flexion reflex pathways. We hypothesize that these changes would be manifested as progressive increases in reflex response with repetitive stimulus application (i.e., "windup") of the flexion reflexes. We investigated the windup of flexion reflex responses in 12 individuals with complete chronic SCI. Flexion reflexes were triggered using trains of electrical stimulation of plantar skin at variable intensities and inter-stimulus intervals. For threshold and suprathreshold stimulation, windup of both peak ankle and hip flexion torques and of integrated tibialis anterior electromyographic activity was observed consistently in all patients at inter-stimulus intervals < or =3 s. For subthreshold stimuli, facilitation of reflexes occurred only at intervals < or =1 s. Similarly, the latency of flexion reflexes decreased significantly at intervals < or =1 s. Patients that were receiving anti-spasticity medications (e.g., baclofen) had surprisingly larger windup of reflex responses than those who did not take such medications, although this difference may be related to differences of spasm frequency between the groups of subjects. The results indicate that the increase in spinal neuronal excitability following a train of electrical stimuli lasts for < or =3 s, similar to previous studies of nociceptive processing. Such long-lasting increases in flexion reflex responses suggest that cellular mechanisms such as plateau potentials in spinal motoneurons, interneurons, or both, may partially mediate spinal cord hyperexcitability in the absence of descending modulatory input.

Neuronal bursting induced by NK3 receptor activation in the neonatal rat spinal cord in vitro

Intracellular recording from lumbar motoneurons and extracellular recording from ventral roots of the neonatal rat isolated spinal cord were used to study the mechanisms responsible for the excitation mediated by NK3 tachykinin receptors. The selective NK3 agonists senktide or [MePhe7]neurokinin B induced a slow depolarization with superimposed oscillations (mean period +/- SD was 2.8 +/- 0.8 s) that, in the majority of cases, showed left-right alternation at segmental level and were synchronous between L2 and L5 of the same side. During agonist wash out (5-20 min) a delayed form of hyperexcitability emerged consisting of bursts lasting 8 +/- 2 s (average interburst interval 55 +/- 21 s) with superimposed oscillations usually with homosegmental alternation and heterosegmental synchronicity. Such bursting was accompanied by depression of GABAergic dorsal root potentials evoked by dorsal root stimulation and of the recurrent inhibitory postsynaptic potential recorded from motoneurons. Despite bursting, motoneuron membrane potential returned to baseline while input resistance was increased. Bursts were a network-dependent phenomenon triggered by previous NK3 receptor activation because bursting was suppressed by glutamate receptor antagonists and was insensitive to motoneuron membrane potential or subsequent application of an NK3 receptor antagonist. NK3 receptors operated synergistically with N-methyl-D-aspartate (NMDA) and 5-hydroxytryptamine (5-HT) to trigger fully alternating locomotor-like rhythms while NK3 receptor antagonism disrupted the same rhythm. In summary, in the neonatal rat spinal cord NK3 receptors could trigger rhythmic activity predominantly with alternation at segmental level but with synchronous coupling between ipsilateral motor pools. NK3 receptor activation could also facilitate fictive locomotor patterns induced by NMDA and 5-HT.

Depression of windup of spinal neurons in the neonatal rat spinal cord in vitro by an NK3 tachykinin receptor antagonist

The effects of the NK3 tachykinin receptor antagonist SR 142801 on synaptic transmission and spike windup induced by trains of stimuli applied to a dorsal root were investigated with intra- and extracellular recording from the neonatal rat spinal cord in vitro. SR 142801 (10 microM) reduced the depolarization (recorded from lumbar ventral roots) induced by senktide (an NK3 agonist) more strongly than the one evoked by substance P methyl ester (SPMeO; an NK1 agonist). Nevertheless, after a long (>2 h) application time, SR 142801 largely depressed the response to SPMeO as well. When NK1 or NK3 receptors were blocked by >50% in the presence of SR 142801, there was also a significant reduction in the cumulative depolarization induced by repeated stimuli to a single dorsal root. This blocking action by SR 142801 was also observed in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist D-aminophosphonovalerate (APV) and the calcium channel blocker nifedipine. Intracellular data from lumbar motoneurons showed that the spike windup was the first and most sensitive target for the SR 142801 blocking effect. Increasing stimulus strength to dorsal root fibers could partly surmount such a block. SR 142801 per se had no direct action on fast synaptic transmission, membrane potential, or input resistance. These findings indicate that SR 142801 could lead to an early, large reduction in the windup of action potential discharge by motoneurons, suggesting its ability to suppress the reflex component of central sensitization evoked by repeated dorsal root stimuli.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

The tachykinin NK1 receptor antagonist GR205171 abolishes the retching activity of neurons comprising the central pattern generator for vomiting in dogs

Tachykinin NK1 receptor antagonists are known to act centrally and to have broad-spectrum antiemetic effects, but their precise site of action has not yet been defined. To identify this site, the effects of the NK1 receptor antagonist GR205171 on the activities of neurons comprising the central pattern generator (CPG) for vomiting were observed in decerebrate paralyzed dogs. A non-respiratory neuron in each of nine dogs was considered to be a CPG neuron based on its response to abdominal vagal stimulation, its location in the CPG area in the reticular formation dorsomedial to the retrofacial nucleus, its firing patterns in prodromal and retching phases and its response to apomorphine. In response to vagal stimulation at 3-10 Hz, the firing of these neurons transiently increased at the onset of stimulation (fast component), gradually increased again (slow component), and finally developed into rhythmic bursts synchronous with retching bursts of the phrenic and abdominal muscle nerves. GR205171 (25-50 microg/kg, i.v.) abolished the slow component and retching bursts in the neurons, and the retching activities of both nerves, but did not change the fast component. The responses of these neurons to repetitive pulse-train vagal stimulation exhibited a vigorous 'wind-up' and finally developed into retching bursts. Both the 'wind-up' phenomenon and retching bursts disappeared after the application of GR205171. These results suggest that the site of the antiemetic action of NK1 receptor antagonists is located in the CPG or in the pathway connecting the solitary nucleus to the CPG.

Effects of a tachykinin NK3 receptor antagonist, SR 142801, studied in isolated neonatal rat spinal cord

Effects of a nonpeptide tachykinin NK3 receptor antagonist, SR 142801, were studied in the isolated spinal cord preparation of the neonatal rat. Potential changes were recorded extracellularly from a lumbar ventral root. Bath-application of neurokinin B induced a dose-dependent depolarization of the ventral root. SR 142801 caused rightward shifts of the concentration-response curve for neurokinin B with pA2 of 6.57, but did not affect the depolarizing responses to other agonists. Stimulation of a dorsal root evoked in the ipsilateral ventral root of the same segment monosynaptic and polysynaptic reflexes of fast time course which were followed by a slow depolarization (ipsilateral slow ventral root potential). SR 142801 depressed the ipsilateral slow ventral root potential. The present results indicate that SR 142801 is a specific antagonist for tachykinin NK3 receptors in the spinal cord and suggest that NK3 receptors are involved in primary afferent-evoked nociceptive responses of spinal neurones.

The tachykinin NK1 receptor antagonist GR205171 prevents vagal stimulation-induced retching but not neuronal transmission from emetic vagal afferents to solitary nucleus neurons in dogs

Tachykinin NK1 receptor antagonists injected into the medulla oblongata are known to abolish vomiting induced by vagal afferent stimulation. Emetic vagal afferents have been shown to synapse with neurons in the medial solitary nucleus (mNTS), which suggests that substance P is a transmitter in the synapse. To examine this possibility, the effects of GR205171, an NK1 receptor antagonist, on retching and mNTS neuronal responses to the stimulation of abdominal vagal afferents were investigated in decerebrate dogs. GR205171 (0.05-0.7 mg kg-1, i.v.) abolished retching induced by either vagal or mNTS stimulation within 5 min. Firing of mNTS neurons in response to pulse-train and sustained vagal stimulation did not change even after the abolition of retching. Similarly, GR205171 did not have any effects on mNTS evoked potentials induced by pulse-train vagal stimulation. In about 20% of mNTS neurons, the peak firing frequency was facilitated to about 150% with repetitive pulse-train vagal stimulation. This facilitation remained even after the abolition of retching. Administration of GR205171 (1 mg ml-1, 30 microliters) into the 4th ventricle abolished retching, with latencies in excess of 120 min These results suggest that substance P does not participate in synaptic transmission between emetic vagal afferents and mNTS neurons in dogs.

Effects of RPR 100893, a potent NK1 antagonist, on the jaw-opening reflex in the guinea pig

RPR 100893 appears as a new potent NK1 selective non-peptide antagonist both in vitro and in vivo, and exhibits high affinity for guinea pig and human NK1 receptor [M. Tabart, J.-F. Peyronel, Synthesis of RPR 100893, prototype of a new series of potent and selective non-peptide NK1 antagonists: the triarylperhydroisoindolols, Bioorg. Med. Chem. Lett., 4 (1994) 673-676.]. Intra-oral administration of RPR 100893 (3, 15, 10, 30 mg/kg) was performed in freely moving guinea pigs during recording of the short- (6-10 ms) and long-latency (18-26 ms) jaw-opening reflex (JOR) elicited by electrical stimulation (0.5 Hz) of the lower incisor tooth pulp. RPR 100893 induced a noticeable and dose-dependent increase of the long-latency reflex thresholds (P<0. 001) but was ineffective on the short-latency responses (P=0.14). The results suggest that, in guinea pigs, the long-latency JOR requires activation of NK1 receptors, while the earlier reflex component, elicited by activation of periodontal afferents, does not. These NK1 receptors could be located either on JOR interneurons activated by tooth pulp afferents or on digastric motoneurons, receiving the tooth pulp input through a polysynaptic pathway.

Sensitization of pain pathways in the spinal cord: cellular mechanisms

Sensitization is manifested as an increased response of neurones to a variety of inputs following intense or noxious stimuli. It is one of the simplest forms of learning and synaptic plasticity and it represents an important feature of nociception. In the spinal cord, repeated stimulation (at constant strength) of dorsal root afferents including nociceptive C fibres can elicit a progressive increase in the number of action potentials generated by motoneurones and interneurones. This phenomenon is termed "action potential windup" and is used as a cellular model of pain sensitization developing at the level of the central nervous system. Understanding the mechanisms responsible for windup generation might allow clarification of the cellular mechanisms of pain signalling and development of new strategies for pain treatment. Action potential windup is observed in a minority of cells only, indicating that certain cell-specific mechanisms are responsible for its generation. The most reliable index to predict windup generation is the rate at which the membrane potential is depolarized during repetitive stimulation. This phenomenon has been proposed to be due to gradual recruitment of NMDA receptor activity, to summation of slow excitatory potentials mediated by substance P (and related peptides) or to facilitation of slow calcium channels by metabotropic glutamate receptors. Little is known about the role of synaptic inhibition in windup, although it should not be underestimated. Each theory per se is unable to account for all the experimental observations. Since NMDA receptors are involved in many forms of synaptic plasticity, additional mechanisms such as summation of slow peptidergic potentials, facilitation of slow Ca2+ currents and disinhibition are proposed as necessary to impart specificity to pain-induced sensitization. These additional mechanisms might be species specific and change during development or chronic pain states.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.

The potency of the novel tachykinin receptor antagonist CGP49823 at rat and gerbil motoneurones in vitro

The novel tachykinin receptor antagonist CGP49823 ((2R,4S)-2-benzyl-1-(3,5-dimethylbenzoyl)-4-(quinolin-4-y lmethylamino)piperidine) has been compared with three other selective non-peptide tachykinin NK1 receptor antagonists. The drugs were tested as antagonists of the depolarization of spinal motoneurones induced by bath application of the selective tachykinin NK1 receptor agonist septide-(6-11) (300 nM) for 120 s at 15 min intervals. The antagonists were bath applied and the depolarizations were recorded from lumbar ventral roots of 7 to 12 day old rat and gerbil hemisected spinal cords in vitro. The gerbil preparation is considered to model the human species variant of the tachykinin NK1 receptor. With the exception of SR140333 ((S)-1-[2-[3-(3,4-dichlorophenyl)-1-[[3-(1-methylethoxy)phenyl]ace tyl]-3-piperidinyl]ethyl]-4-phenyl-1-azoniabicyclo[2.2.2]octane chloride), the antagonists were approximately thirty-fold more potent on gerbil preparations. The respective mean IC50 values from gerbil preparations produced by CP96345 ((2S-cis)-2-(diphenylmethyl)-N-[(2-methoxyphenyl)methyl]-1-azabicy clo[2.2.2]octan-3-amine), CGP49823, SR140333 and CP99994 ((2S-cis)-N-[(2-methoxyphenyl)methyl]-2-phenyl-3-piperidinamine) were, in microM +/- S.E. (n) 0.10 +/- 0.02 (6), 0.22 +/- 0.03 (6), 0.30 +/- 0.10 (5) and 0.38 +/- 0.02 (5) and the corresponding values from the rat preparations were 3.7 +/- 0.4 (5), 7.8 + 1.3 (5), 1.06 +/- 0.16 (6) and 10.5 +/- 2.2 (7). Dominance of tachykinin NK1 receptor activity in the measured responses was confirmed by low potency of the tachykinin NK2-selective antagonist SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl] benzamide) which yielded an IC50 value of 12.0 +/- 2.8 (5) on gerbil preparations and produced less than 50% depression of septide-induced depolarization of rat motoneurones at the highest concentration (100 microM) tested.

NMDA receptor-independent mechanisms responsible for the rate of rise of cumulative depolarization evoked by trains of dorsal root stimuli on rat spinal motoneurones

The mechanisms responsible for the rate of rise (RR) of cumulative depolarization induced by dorsal root stimulus trains were investigated with intracellular recordings from motoneurones of the rat isolated spinal cord. The NMDA receptor antagonists CPP or APV depressed the cumulative depolarization but not its RR which could still be fast enough to elicit action potential wind-up. RR size was correlated with a slow synaptic potential (detected in CPP or APV solution) with which it shared similar voltage dependence. The NK1 antagonist SR 140333 depressed cumulative depolarization, RR and slow synaptic potentials. It appears that the RR (and the ability to express wind-up) was determined by summation of slow synaptic potentials partly mediated via activation of NK1 receptors.

Membrane potential oscillations of neonatal rat spinal motoneurons evoked by electrical stimulation of dorsal root fibres

Intracellular recording from lumbar motoneurons of the neonatal rat isolated spinal cord bathed in standard saline solution was used to study membrane potential oscillations which accompanied the decay phase of excitatory postsynaptic potentials (EPSP) induced by single electrical pulses to an adjacent dorsal root. About 60% of motoneurons displayed rhythmic oscillations of 10 +/- 2 mV maximal amplitude and 7 +/- 0.5 Hz frequency. Ability to generate oscillations could not be correlated to the cell membrane properties or to the age of the preparation (5-13 days). The oscillation frequency was independent of membrane potential (-100 to -45 mV) or of the intensity of dorsal root stimuli. The oscillation amplitude was linearly related to the cell potential within the same voltage level. Fast Fourier transform analysis showed that the power spectrum of oscillations peaked at approximately 8 Hz. Electrically evoked activities and spontaneous events displayed similar cut-off frequencies. When the cell membrane potential was steadily depolarized, a hyperpolarizing pulse applied during the decay phase of the EPSP promptly revealed the presence of oscillatory behaviour. Pharmacological block of neurokinin-1 or N-methyl-D-aspartate receptors depressed the decay phase of the EPSP and the associated oscillatory responses. It is suggested that rhythmic oscillations were probably due to summated synaptic potentials generated at the premotoneuron level and are perhaps of functional relevance to motoneuron behaviour.